Revised internal pressure calculation in AS/NZS 1170.2:2021 Part 2

The local pressure factor is ment to represent the external local pressure factor acting on the opening at the max event. There was a fair bit of testing done and a thesis was born and it does suggest that you are correct, the local variable pressure is still present with an opening. I think the guy was Mitchell.

However what the code didn't take from the testing was the flexibility of the structure.

Do you have any references? I can't find anything by Mitchell?

You mean this video:

rowingengineer said:

I thought it was Chris h on the NZ video where we asked but I can't find it ont hat video, but we asked one of the committee members during the presentations to clarify the intent.

Click to expand...

The question was asked here too, but noanswer wasn't given:

Tomfh said:

What do you mean from how physics would work? I’m trying to understand, but it seems contradictory with what local pressures represent.

Local pressure factors represent short, localised spikes due to turbulent corner flow, enough to damage cladding, but not the structure. So how can that same brief pressure spike (say kL= 1.5,2,3), once inside, load the entire internal volume and frame? Where does the extra energy come from to do all that work, if it wasn’t present to begin with? Do the flow dynamics change radically if there’s an opening, with the local pressures persisting in the location of openings?

Click to expand...

My gut feeling is to agree with you. Though my understanding is that local pressure are not necessary all "brief pressure spikes" eg corners. But I still generally agree with your thesis

However I have questions:
-Would air flow as is required to increase the pressure inside the building be sufficient and not affect the actual pressure at the local source? Unlikely IMO/
-Would the volume of the airflow required be sufficient to given buildings are leaky and cladding is flexible?

I think applying local pressure factors to the entire internal pressure would be a radical change in the loading and in the design of buildings. Based on my experience, looking back at historical design from the last 70 years, I have not seen buildings have random catastrophic failure that would be expected with this drastic increase in unanticipated loading. But just my 2 cents.

Euler,
Some emotional words, radical, catastrophic, I doubt that anyone would be able to pinpoint any of the building failures to internal being under appreciated by the difference of this calc.

Overall the change to the internal pressure with local pressure changes hasn't greatly changed the outcomes, the wind ward local pressure factore is 1.5 and the area is 0.25a2, for a 12x12x3.5 that is an a of 3.5, which gives an area 2.6m2, and if you opening is greater than this area in the example building than that KL is 1. They you compare this to background permeability you will be down the table, and a larger opening like a roller door will govern, that is unaffected by KL.

Internal suction, well that is a different story, it is higher generally, however I don't find this changes the worst design Case of CW + IP.



Tomfh,
Overall I don't find the outcomes significantly different, just a few extra calculations to show the larger opening is the governing rather than the small 0.25a2 and has a slight lower internal pressure due to kV.....


Overall I would encourage people to do the calcs and see if it really has a impact. A group of North Qld engineers spent a heap of time doing different setups of existing buildings to see if they could be certified as as1170.2-2021 and came to be conclusion that for most building the 0.7 was still a conservative/acceptable assumption for internal pressure coefficient. This was in region C.

rowingengineer said:

Overall I don't find the outcomes significantly different, just a few extra calculations to show the larger opening is the governing rather than the small 0.25a2 and has a slight lower internal pressure due to kV.....

Click to expand...

With small openings, kL can be 2 or even 3, which makes a huge difference. Applying a CPe of 0.9 with kL equal to 3 results in massive internal pressures, apparently acting across the entire internal surface.

What is the research showing it caps out at 0.7? That would help us a lot.